Cloud condensation nuclei (CCN) activity and composition of secondary organic material

 

Authors

Scot T. Martin — Harvard University
Qi Chen — Peking University
Mikinori Kuwata — Earth Observatory of Singapore, NTU

Category

Aerosol Properties

Description

Dependence of Korg on O:C ratio. Lines are drawn through the data at each thermodenuder temperature to guide the eye. Marker shape represents M prior to heat treatment. Marker color represents thermodenuder temperature.
The effects of thermodenuder treatment on CCN activity and composition of organic particles grown by α-pinene ozonolysis were investigated. The secondary organic material (SOM) was produced in a continuous-flow chamber, with steady-state organic particle mass concentrations M ranging from 1.4 to 37 μg m-3. Particles exiting in the outflow were heated to temperatures T of up to 100 °C in a thermodenuder. The oxygen-to-carbon (O:C) and hydrogen-to-carbon (H:C) ratios were measured on-line. The observed elemental ratios were fit by a linear function, given by (H:C) = -0.8 (O:C) +1.8 for 0.38 < O:C < 0.50. This fit included the dependence on both M and T. The implication is that a single variable, specifically M following thermodenuder treatment, served as an accurate predictor for O:C(M(T)) and H:C(M(T)). This result suggests that equilibrium partitioning largely governed the initial volatilization in the thermodenuder. By comparison, the CCN activity had a different dependence on thermodenuder treatment. At 25 °C, the CCN activity was independent of M, having an effective hygroscopicity parameter Korg of 0.103 ± 0.002. At 100 °C, however, Korg varied from 0.105 for 1.4 μg m-3 to 0.079 for 37 μg m-3, indicating that for high mass concentration, the CCN activity decreased with heat treatment. The interpretation is that the oligomer fraction of the SOM increased at elevated T, both because of particle-phase reactions that produced oligomers under those conditions and because of the relative enrichment of lower-volatility oligomers accompanying the evaporation of higher-volatility monomers. Oligomerization reactions increase the effective molecular weight, thereby significantly influencing CCN activity. The types and rates of oligomerization reactions that occur depend strongly on the types and concentrations of functional groups present, which in turn are strongly influenced by M. We conclude with a hypothesis, which is supported by a detailed molecular kinetic model, that the changes in Korg at high T were more significant at high M compared to low M, because particle-phase SOM at high M contained a mix of functional groups favorable to oligomerization, such as carbonyl groups. The oligomerization reactions at elevated T might serve as a laboratory-accelerated model system for similar reactions that occur in the atmosphere at longer timescales, such as timescales of days to weeks not easily accommodated directly in laboratory studies.